14-sqlite's primary KEY and UNIQUE constraint

First, PRIMARY key

uniquely identifies a single row ( only one primary key in a table )

The

primary key should be meaningless to the user ( commonly used for indexing )

never update the primary key , or violate the principle of no meaning to the user

the primary key should not contain dynamically changing data, such as timestamp, creation time column, modified time column, etc.

In a two-table relationship, the primary key is used to refer to a specific record in one table from another table

Syntax:

CREATE TABLE table name (column name 1 data type primary key, column name 2 data type, column name 3 data type, ...);

Second, the only constraint

used to guarantee that data in a column (or set of columns) is unique, similar to a primary key, but that a table with a primary key can contain multiple unique constraints, but only one primary key unique constraint column can be modified or updated

When you create a table, you set it by unique

Syntax:
CREATE TABLE table name (column name 1 data type unique, column name 2 data type,..。 );

Third, examples

14-sqlite's primary KEY and UNIQUE constraint